Abrasive articles typically comprise a substrate or backing having on a surface thereof a plurality of abrasive particles secured thereto by a binding medium. In some instances coated abrasives include a backing, a first coating layer bonded to one side of the backing (commonly referred to as a make coating), at least one layer of abrasive particles bonded to the backing using the first coating layer, and a second coating layer overlaying the abrasive particles, which is commonly referred to as a size coating. The purpose of the size coating is to reinforce retention of the abrasive particles. Another type of coated abrasive construction includes a backing having an abrasive composite bonded to one side of a backing. The abrasive composite includes a plurality of abrasive particles dispersed throughout a binding medium. Typically, the abrasive composite is formed from an abrasive slurry.
Coated abrasive articles can be converted into a wide variety of different forms such as belts, discs, cones and sheets. It is sometimes preferable to have a pressure sensitive adhesive (PSA) coating on at least a portion of the non-abrasive side of the coated abrasive. The coated abrasive, for example, a disc, can then be secured to a support pad and when the abrasive disc is to be replaced, it can be removed and a new abrasive disc secured to the same support pad.
A common way to package a plurality of coated abrasive discs each having a PSA backing is illustrated in U.S. Pat. No. 3,849,949 (Steinhauser et al.). Steinhauser discloses an abrasive disc product comprising a convolutely wound concatenation of coated abrasive discs having disposed on the side opposite the coated abrasive a PSA with the adhesive coating placed in releasable contact with the abrasive coating. Unfortunately, while these constructions are adequate for most purposes, this direct contact between the abrasive coating and PSA can facilitate transfer of the PSA to the abrasive coating, especially in the finer abrasive particle grades of coated abrasives. Consequently, this transfer may cause contamination of the workpiece and the abrasive coating, and may reduce the adhesive nature of the PSA coating, which may present a safety concern. In addition, the force required to unwind the continuous roll of Steinhauser, while not prohibitive, may be an inconvenience for the user. This is compounded by the propensity of many PSA's to increase in adhesion with dwell time on a surface, translating into even higher unwind force.
The PSA transfer problem associated with this type of construction occurs if the "peel adhesion" between the PSA and abrasive side of the coated abrasive is greater than the internal cohesive threshold strength of the PSA. If this is true, the PSA can split and partially transfer to the abrasive coating side (PSA transfer). The problem is particularly acute in the finer abrasive particle grades of electrostatically coated abrasives since the finer grades have an increased surface area. This results in a higher peel adhesion for removal of PSA from the abrasive side of the coated abrasive.
One solution to the adhesive transfer and high unwind force problems is to have a paper or plastic release liner placed between the PSA coating and abrasive grains, as disclosed in U.S. Pat. No. 3,267,623 (Block). However, this becomes an additional expense for the customer and the liner must be disposed of. Additionally, where the coated abrasives are used to condition a surface for painting, and where the liner has a silicone-based coating, the silicone may be contaminate the unpainted surface, and may cause the paint to "bubble". Thus, many users prefer that PSA type coated abrasive discs do not have a liner (especially silicone-based) associated with them.
Coated abrasive articles are used to abrade a wide variety of substrates, including wood, wood like materials, plastics, fiberglass, soft metal alloys, enamel surfaces, and painted surfaces. One problem, to all of these different substrates is "loading" or clogging. Loading is the industry term that describes the phenomenon of particles from the workpiece being abraded becoming lodged in between the abrasive particles. Loading reduces the cutting ability of the abrasive article, and thus the useful life of the abrasive article is substantially reduced.
In an attempt to overcome this loading problem, Twombley, in U.S. Pat. No. 2,768,886, discloses the use of a metal stearate, metal palmitate or metal laurate coating over the abrasive surface. One theory for the success of these compounds is that this type of organic metal coating powders off of the abrasive surface, which in turns causes the abraded particles thus loaded to also powder off the abrasive surface, thus reducing loading.
Although abrasive articles which include anti-loading coatings of zinc stearate and other organic metal coatings have had wide commercial success, their use in some applications has the disadvantage in that such coatings are not coherent as in the case of a polymer and have a tendency to flake from the abrasive surface, and the flaked residue finds its way onto surfaces which the user would rather not have contaminated.
European Pat. App. 0 433 031 A1, Stubbs, published Jun. 19, 1991, describes abrasive elements having between the particles of abrasive material various fluorochemicals selected from two classes: compounds having fluorinated aliphatic group attached to a polar group or moiety, and fluoropolymers having a molecular weight of at least 750 and comprising a non-fluorinated polymeric backbone having a plurality of pendant fluorinated aliphatic groups.
U.S. Pat. No. 2,881,064, Rankin et al., discloses an organosilicone-based supersize coating which apparently prevents coated abrasives from loading. Examples of organosilicone supersized coatings include that comprised of dialkoxy diamino silane, a mixture of chloro and methyl silanes, the thermal setting organo-silicone resin containing a plurality of aromatic groups attached to silicon atoms, and highly polymerized dimethyl siloxane polymers.
U.S. Pat. No. 3,042,508, Hagis, teaches a metal-backed abrader having cutters spaced over the surface thereof and a supersize coating consisting essentially of a fluoroethylene polymer which apparently reduces loading.
U.S. Pat. No. 3,043,673, Klein, concerns a coated abrasive having an oxy-containing compound (for example aliphatic polyhydric alcohols or aliphatic polyethers) in the size resin to reduce loading.
U.S. Pat. No. 3,795,496, Greenwood, discloses a coated abrasive having a plasticized polyvinyl acetate supersize coating which apparently reduces loading.
U.S. Pat. No. 2,202,765, Guth, pertains to a coated abrasive having a size coating that is loading resistant, comprising the reaction product of polyhdyric alcohols or their anhydrides with other compounds with which they are adapted to react.
U.S. Pat. No. 2,532,011, Dahlquist et al., pertains to a pressure sensitive adhesive tape that contains a low adhesion backside coating of a polyvinyl carbamate.
U.S. Pat. No. 4,988,554 (Peterson et al.) teaches a coated abrasive article which contains a lithium salt of a fatty acid supersize to reduce the amount of loading. On the nonabrasive side is a pressure sensitive adhesive coating. When the coated abrasive is packaged, the lithium salt of the fatty acid apparently prevents significant transfer of the pressure sensitive adhesive to the abrasive grains.
U.S. Pat. No. 4,973,338 (Gaeta et al.) pertains to a coated abrasive that has improved anti-static, lubricity and antiloading properties. The coated abrasive has a supersize coating comprising a quaternary ammonium compound, which has from about 15 to 35 carbon atoms and a molecular weight not less than about 300. Examples of the quaternary ammonium compounds include (3-lauramido-propyl)trimethylammonium methyl sulfate, stearamidopropyldimethyl-betahydroxyethylammonium nitrate, N,N-bis(2-hydroxyethyl)-N-(3"-dodecyloxy-2"-hydroxypropyl)methylammonium methylsulfate and stearamidopropyl-dimethyl-betahydroxyethylammonium dihydrogen phosphate.
Other patents of interest include U.S. Pat. Nos. 3,869,834; 4,720,941; 5,061,294; 4,563,539; 4,359,369; 4,554,339; 4,597,987; 4,313,988; 4,822,687; 4,269,963; 4,743,474; 4,530,882; and 4,525,566. Practical difficulties in processing silicones are discussed in Huettner, D. J., in a conference paper entitled "Moisture Curing Silicone Release Coating Technology; A Coating Process is the Missing Component", presented at the 1988 Pressure Sensitive Tape Council Technical Seminar.
Of the above-mentioned constructions, while some are concerned with loading resistant coatings and some with transfer of adhesive to the abrasive surface, the finer grades of coated and other abrasives continue to suffer reduced performance due to adhesive transfer.